The present invention relates to a length or angle measuring instrument of the type which includes a scale and a scanning unit adapted to scan the scale, wherein the scanning unit comprises a scanning plate which comprises at least first and second scanning fields, phase shifted with respect to one another for generating respective first and second scanning signals which are likewise phase shifted with respect to one another, and means for directing light at the scanning plate.
In one known length measuring system a scanning unit is provided which operates to generate two analog scanning signals which are offset in phase angle with respect to one another. In this known device, the desired phase difference between the two analog signals is set by applying one of the two analog signals to a first input of an amplifier mixing stage and applying a regulating voltage to the second input of this amplifier mixing stage, which second input is derived from the other analog signal and is adjustable by means of electrical setting members. This arrangement requires a number of electrical components, and is both relatively expensive and susceptible to errors due to failure of the respective electrical components. Moreover, the phase angle adjustment is lost when the evaluating arrangement which contains the relevant electrical components is changed.
It is also a known practice in photoelectric position measuring systems to regulate the amplitude of analog scanning signals by using screws to control the flux of light which passes through the scanning fields of the scanning unit.